Magnetic Properties of Diaminoglyoxime Complexes with a Slipped

Magnetic Properties of Diaminoglyoxime Complexes with a Slipped-Stack Structure (2,3-Diamino-1,4-diazabuta-1,3-diene-1,4-diol)bis(2,3-diamino-4-hydrox...
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J. Phys. Chem. B 1997, 101, 4498-4507

ARTICLES Magnetic Properties of Diaminoglyoxime Complexes with a Slipped-Stack Structure (2,3-Diamino-1,4-diazabuta-1,3-diene-1,4-diol)bis(2,3-diamino-4-hydroxy-1,4-diazabuta1,3-dien-1-olato)MII (M ) Co, Ni, and Cu) Takahiro Hirotsu,* Akinari Sonoda, Hirofumi Kanoh, and Kenta Ooi Shikoku National Industrial Research Institute, 2217-14 Hayashi-cho, Takamatsu 761-03, Japan

Manabu Senoˆ Department of Industrial Chemistry, College of Science and Technology, Nihon UniVersity, 1-8 Kanda-Surugadai, Chiyoda-ku, Tokyo 101, Japan ReceiVed: September 24, 1996; In Final Form: March 12, 1997X

The magnetic properties of the divalent metal complexes [ML2(HL)] (M ) CoII, NiII, and CuII; HL ) 2,3diamino-1,4-diazabuta-1,3-diene-1,4-diol, i.e., diaminoglyoxime) with a slipped-stack structure were examined. The magnetic susceptibilities and the electron spin resonance spectra indicate that [CoL2(HL)] exhibits a ferromagnetic interaction along the stack chain, the strength of which is estimated to be 13.5 K, while [CuL2(HL)] is paramagnetic and [NiL2(HL)] is diamagnetic. The magnetic behaviors of the [ML2(HL)] complexes can be predicted precisely by comparing the spin splittings of the partially occupied molecular orbitals of the monomer complexes ML2 and the dimer complexes 2ML2 which are equivalent in structure to the relevant moieties in the real stack chains, based on molecular calculations using the DV-XR method. The modes of the differential spin densities for the dimers 2ML2 strongly suggest that the principal mechanism of superexchange on [CoL2(HL)] may operate through strong interaction of the 3dzx orbitals of Co atoms with the 2pz orbitals of two apical N atoms which form the πz-type systems of the adjacent complex molecules along the stack chain.

Introduction Advanced understanding of magnetostructural correlations in metal complexes has been largely derived from intensive study of molecular-based materials exhibiting spontaneous magnetization. Kahn et al. have demonstrated that the MnII-CuII bimetallic chain compound with metals bridged by 2-hydroxy-1,3-propylenebis (oxamato) exhibits spontaneous magnetization at 4.6 K,1a on the basis of the notion that heteropolymetallic chain systems, even if the interaction between nearest neighbors is antiferromagnetic, never fail to fall into magnetic ground states owing to noncompensation of the local spins.1b Gatteschi et al. have found alternant MnII(hfac)-nitronyl nitroxide radical (NIT-R) linear chain complexes (hfac ) hexafluoroacetylacetonate, NIT-R ) 2-substituted 4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide; R ) ethyl,2a n-propyl,2a and isopropyl2b) with spontaneous magnetization at 7.6-8.6 K by predicting the coupling of a nitronyl nitroxide directly bound to a metal ion from the overlap mode between the molecular orbitals with an unpaired electron of the ligand and the metal ion.2c On the basis of such a design strategy using through-bond magnetic interactions to obtain the molecular-based magnets, a two-dimensional spin network composed of MnII(hfac) and a trinitroxide radical having three ligating sites and a quartet ground state has already been synthesized and it exhibits a critical temperature of 3.4 K.3 At present, the synthesis of three-dimensional oxalatebridging networks is being explored extensively to obtain a * Author to whom correspondence should be addressed. Fax: 81-87869-3550. E-mail: [email protected]. X Abstract published in AdVance ACS Abstracts, April 15, 1997.

S1089-5647(96)02944-6 CCC: $14.00

higher critical temperature for spontaneous magnetization.4 On the other hand, on the basis of a design strategy using throughspace magnetic interactions in molecular assemblies with stacked and layered modes,5 Miller et al. have reported that a few charge-transfer complexes such as [Fe(C5Me5)2] (TCNE)6a and [V(TCNE)x] [y(CH2Cl2)]6b (Me ) methyl, TCNE ) tetracyanoethylene) are molecular-based magnets. Oshio et al. have described three-dimensional organic radical networks with paramagnetic metal complexes as a network component having magnetic interactions with each other through the radical network.7 However, the mechanism of magnetic interactions between spin centers through molecular columns has not been made sufficiently clear from the molecular-orbital method. This probably makes the design of molecular-based magnets by the through-space approach difficult, but this approach is a more useful strategy, particularly for design of organomagnetic compounds,8 rather than the through-bond approach. We have been interested in divalent-metal complexes with diaminoglyoxime [ML2(HL)] (M ) Co,9 Ni,10 and Cu;11 HL ) 2,3-diamino-1,4-diazabuta-1,3-diene-1,4-diol, i.e., diaminoglyoxime) as a system underlying magnetic interactions in a stacked molecular chain, because these compounds are monoclinic, they belong to space group C2/c, and because their structures consist of chains of planar MIIL2 units stacked along the b-axis through long M-N bonds to terminal N atoms of the adjacent molecular units; the stacked chains form sheets through hydrogen bonding with neutral HL molecules in the {002} plane for the CoII compound9 and in the {201} plane for the NiII 10b and the CuII compounds.11 Recently we have found that the CoII compound [CoL2(HL)] exhibits ferromagnetic interactions along the stack © 1997 American Chemical Society

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Figure 1. The coordinate and numbering of atoms of the monomers ML2 (left) and the dimers 2ML2 (right). The hydrogen atoms H10 (H37) and H23 (H50) of the oxime groups are bound to the oxygen atoms O7 (O34) and O20 (O47), respectively, in the M ) Co systems as in this figure and conversely, to the oxygen atoms O16 (O43) and O3 (O30), respectively, in the M ) Ni or Cu systems.

chain,12 that the [NiL2(HL)] compound is diamagnetic, and that the [CuL2(HL)] compound is paramagnetic. Both [NiL2(HL)] and [CuL2(HL)] exhibit magnetic properties quite different from that of [CoL2(HL)]. In this study, we deal with the characteristic magnetic behaviors of the compounds [ML2(HL)] (M ) CoII, NiII, and CuII) in terms of their magnetic susceptibility. Also, we discuss magnetic interactions between the monomer units in the dimeric model complex 2ML2 with a structure equivalent to that of a pair of neighboring molecular units in the stack chain of the real system, based on molecular orbital calculations using the unrestricted DV-XR method,13 in order to throw light on the nature of magnetism of the stack chain of [ML2(HL)]. Experimental Section Materials. Diaminoglyoxime (HL) was obtained as colorless pillar-shaped crystals by reacting 0.1 mol of dithiooxamide with 0.2 mol of hydroxylamine in 350 mL of methanol under reflux followed by recrystallization from water with activated carbon for decoloration.14 Anal. Calcd for C2H6N4O2: C, 20.34; H, 5.12; N, 47.44. Found: C, 19.69; H, 5.08; N, 47.88. The cobalt(II) complex [CoL2(HL)] was obtained as dark purple pillar-shaped crystals from a warm aqueous solution (100 mL) containing [CoII(CH3COO)2]‚4H2O (10 mmol) and HL (20 mmol) under nitrogen.9 Anal. Calcd for CoC6H16N12O6: Co, 14.33; C, 17.53; H, 3.92; N, 40.88. Found: Co,14.32; C,17.75; H, 3.79; N, 40.80. The nickel(II) complex [NiL2(HL)] was obtained as brick-red pillar crystals according to the procedure described by Endres et al.10a Anal. Calcd for NiC6H16N12O6: Ni, 14.28; C, 17.54; H, 3.92; N, 40.90. Found: Ni, 14.03; C, 17.90; H, 4.01; N, 40.71. The copper(II) complex [CuL2(HL)] was obtained as brown needle crystals according to the method described by Endres et al.11 Anal. Calcd for CuC6H16N12O6: Cu, 15.28; C, 17.33; H, 3.88; N, 40.42. Found: Cu, 15.11; C, 17.13; H, 3.92; N, 40.55. Magnetic Studies. Magnetic susceptibility data were collected on a Toei Kogyo Model VSM3-15 vibrating-sample magnetometer and a Quantum Design Model MPMS-5S SQUID magnetometer. The magnetic susceptibility was measured on microcrystalline samples of about 70 mg under a field of 1 T in the temperature range 2-300 K. All the observed suscep-

tibilities were corrected for diamagnetism estimated from Pascal's constants.15 ESR spectra were recorded on a JEOL JES-RE3X spectrometer operating at X-band frequencies with a magnetic field modulation of 100 kHz. Microwave frequency was measured with an Advantest TR5212 microwave counter, and the magnetic field values of signals were measured with a JEOL ES-FC4 NMR Gauss meter. Calculation of Molecular Orbitals by the DV-Xr Method. Molecular orbitals of the complex monomers and dimers ML2 and 2ML2 were calculated by the unrestricted DV-XR method16 on a Sony model NWS-5000UF net work station. The structures of each monomer and dimer are equivalent to those of a complex molecule and a pair of adjacent complex molecules in the corresponding stack chain determined crystallographically,9-11 respectively. Thus, because of their Ci symmetry, the coordinate employed for the monomers has its origin at the position of the central metal (M1) and its XY plane on the molecular plane containing four nitrogen atoms (N2, N6, N15, and N19), while that for the dimers has its origin at the center of symmetry and its XY plane parallel to the planes of the monomer units, one with the atoms M1, N2, N6, N15, and N19 and the other with the atoms M28, N29, N33, N42, and N46, as shown in Figure 1. Basis sets are 1s for H, 1s-2p for C, N, and O, and 1s-4p for Co, Ni, and Cu. Accordingly, the number of types of atoms is 14 for the monomers and 27 for the dimers, while the number of types of orbitals is 37 for the monomers and 66 for the dimers. All the calculations were performed using the exchange parameter R fixed at 0.7, until the difference between the input and output values of each atomic orbital population was less than 1 × 10-5 electrons for the monomers and less than 2 × 10-5 electrons for the dimers. Results Magnetic Behaviors. The magnetic behavior of the cobalt(II) complex [CoL2(HL)] is shown as χmT against T in Figure 2a, where χm denotes molar magnetic susceptibility and T denotes temperature. From Figure 2a, the χmT value is 0.540 emu K/mol at 298 K, indicating that its effective magnetic moment µeff is equal to 2.08 µB; µB denotes Bohr magneton. Assuming that the complex is isotropic, the spin quantum

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Figure 2. Temperature dependence of χmT for (a) [CoL2(HL)] and (b) [CuL2(HL)]. The solid line corresponds to the best fit calculated as described in the text.

number S can be estimated from the equation µeff/µB ) g[S(S+1)]1/2, where g is a g-factor. If this isotropic g value is 2.35 as described below, S is equal to 0.52, indicating that the cobalt(II) complex has an S ) 1/2 ground state, i.e., a low-spin state. In Figure 2a, a marked rise in χmT at temperatures lower than 50 K indicates a transition to a phase with dominant ferromagnetic interactions. On the basis of the crystallographic information on [CoL2(HL)], this result could be reasonably interpreted in terms of a dominant ferromagnetic spin-ordering within the stack chain along the b-axis and a secondary antiferromagnetic interaction between the chains in the ab plane. Thus, we assume tentatively the isotropic S ) 1/2 Heizenberg model based on a high-temperature Pade´ expansion (eq 1),17 modified by the addition of a mean-field correction (eq 2) to account for the interchain interactions,18 where K ) J/2kT; J is the ferromagnetic coupling constant, Z is the number of nearestneighbor chains (Z ) 2 in this study), J′ is the exchange integral for the magnetic interaction between the nearest-neighbor chains, and the other symbols have their usual meanings.

χm′T ) (NµB2g2/4k){1.0 + 5.797 991 6K+16.902 653K2+ 29.376 885K3+29.832 959K4 +14.036 918K5)/(1.0+ 2.797 991 6K+7.008 678 0K2+8.653 864 4K3 + 4.574 311 4K4)}2/3 (1) χm(corr))χm′/{1 - (2ZJ′/Ng2µB2)χm′}

(2)

Looking for the minimum of the factor R defined as R ) Σ{(χmT)calc - (χmT)obs}2/Σ{ (χmT)obs}2 by the Gauss-Newton method,19 the parameter values of J, J′, and g were determined to be consistent with the observed results in Figure 2a as follows: J ) 13.5K, J′ ) -0.005K, and g ) 2.35 for R ) 9.36 × 10-3 at temperatures 10-298 K. The theoretical curve given by the solid line in Figure 2a is in good agreement with the experimental data. This indicates the existence of ferromagnetic intrachain interactions along the b-axis accompanied by a very weak interchain antiferromagnetic interaction in the ab plane. The magnetic data of [CuL2(HL)] are plotted as χmT against T in Figure 2b. Because the value of χmT is equal to 0.37 emu K/mol at room temperature, the µeff value is 1.73 µB, which is consistent with S ) 0.48, provided its isotropic g value is equal to 2.064 as obtained by the electron spin resonance (ESR) measurement. In Figure 2b, the χmT value very slightly

Figure 3. ESR spectra of (a) [CoL2(HL)] and (b) [CuL2(HL)] at 173 K.

increases down to ca. 30K, indicating the existence of a very weak ferromagnetic interaction in [CuL2(HL)]. This ferromagnetic interaction is so weak that [CuL2(HL)] is assumed to be paramagnetic in an S ) 1/2 ground state. The observed χm values of [NiL2(HL)] were about -1.49 × 10-4 emu/mol not dependent on temperature, which are almost equal to the calculated value based on diamagnetism from Pascal’s constants: -1.39 × 10-4 emu/mol.20 This result means that the complex [NiL2(HL)] is a usual diamagnetic compound. ESR Spectra. ESR spectra of [CoL2(HL)] and [CuL2(HL)] are shown in Figures 3a and 3b, respectively. The ESR spectrum of [CoL2(HL)] is characterized by the combination of a very broad band centered at an average g value (gav) of 2.35 with a very weak but distinct hyperfine splitting (hfs) pattern centered at a gz value of 2.012, which is often observed in CoII(Hdmg)2 (H2dmg ) butane-2,3-dione dioxime)21 and results from the interaction of the unpaired electron with the 59Co nucleus (I ) 7/ ), with A 2 Co ) 12 mT. The absence of any hfs signal from 14N (I ) 1) nuclei on the apical sites means that the interaction between Co and N atoms is as weak as in the case of CoII(Hdmg)2 in frozen water and alcoholic solvents and much weaker than that observed in strongly coordinating media like pyridine.21 The very broad absorption would be due to the ferromagnetic spin-ordering within the stack chain described above, because the broadening of ESR spectra by such ordering has often been observed on CuII and MnII complexes.22 The ESR spectra of [CoL2(HL)] down to 123 K are almost the same as that shown in Figure 3a, except for a slight increase in intensity with a decrease in temperature. On the other hand, the ESR spectrum of [CuL2(HL)] exhibits a slightly anisotropic absorption band centered at a gav value of 2.064, with a width of peak to peak of 10.24 mT, as shown in Figure 3b. The observed values of gav and the peak to peak width are in good agreement with the result reported by Endres et al.11 The ESR spectra of [CuL2(HL)] down to 123 K are almost the same as that in Figure 3b, except for a gradual increase in intensity with a decrease in temperature. It is noted that the ESR spectrum of [CuL2(HL)] is much sharper than that of [CoL2(HL)] in Figure 3, implying that the complex [CuL2(HL)] has noncoupled spins. The complex [NiL2(HL)] exhibited only very weak bands, probably due to some impurities. MO Calculations by the DV-Xr Method. The calculated one-electron energy level diagrams for the monomers ML2 (M

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Figure 4. Electronic levels of the monomers ML2 and the dimers 2ML2 (M ) CoII, NiII, and CuII). The MOs for up spin (v) and down spin (V) are arranged in the left and right columns for each molecule, respectively. Solid and broken lines denote occupied and unoccupied molecular orbitals, respectively. It does not matter whether the POMOs have up spins or down spins.

) CoII, NiII, and CuII) are shown in Figure 4, where it does not matter whether the unpaired electron is up spin (v) or down spin (V). From Figure 4, the monomers CoL2 and CuL2 are spinpolarized to have an S ) 1/2 ground state, i.e., a low-spin state; on the other hand, the monomer NiL2 is nonspin-polarized to fall in a singlet ground state. The results from Mulliken’s population analysis on these monomers are summarized in Table 1. Every atom on each monomer has the number of electrons consistent with that expected on the basis of electronegativity, considering that the N atoms of amino groups have higher electron densities than those of oxime groups because in the former the nitrogens are directly bound to hydrogen atoms with a lower electronegativity. The CoL2 has the partially occupied molecular orbital (POMO) 38ag. The MO 38agV with an unpaired electron of CoL2 consists of the atomic orbitals of 3dzx (on Co), 2pz (on C, N, and O) and 2px (on N5 and N18), characterized as a π-type MO developing along the Z-axis (Figure 5a). There is little difference between the MO 38agv for up spin and the MO 38agV for down spin, because the monomer CoL2 is a low-spin compound. On the other hand, the MO 39agv with an unpaired electron of CuL2 is an antibonding π system developed on the molecular plane Z ) 0, principally consisting of the 3dxy (Cu), the 2s (C8, C21, N2, N6, N15, and N19), the 2px (N2, N6, N15, N19, and O), and the 2py bases (C8, C21, N2, N6, N15, N19, and O) with a slight superposition of the 2pz bases (N5, N18, O7, and O20) (Figure 5c). There is also little difference between the MO 39agv and the MO 39agV for CuL2. The highest occupied molecular orbital (HOMO) 37au of NiL2 is a π-type MO, consisting of mainly 2pz atomic orbitals of the ligands L2 with little contribution from atomic orbitals on the central Ni (Figure 5b). The differential spin densities (∆FvV) of CoL2 and CuL2 are shown in Figure 6, where ∆FvV ) Fv - FV; Fv and FV are spin densities of up spins and down spins, respectively. The monomer CoL2 has a differential spin density delocalized over the whole molecule and developed along the Z-axis; thus it possesses a large ∆FvV value of -0.068 28e even on N5 and

N18 at apical sites of Co atoms of the neighboring complex molecules in the stack chain (Table 1). On the other hand, the monomer CuL2, being quite different from CoL2, has a localized differential spin density predominantly on the central Cu atom and the nitrogen (N2, N6, N15, and N19) and oxygen atoms surrounding the Cu atom, with very low differential spin densities on N5 and N18 atoms. Moreover, the differential spin density of CuL2 develops predominantly on its molecular plane Z ) 0. Accordingly, the difference in ∆FvV between CoL2 and CuL2 strongly suggests that the magnetic interactions between neighboring molecules in the stack chain are stronger in the complex [CoL2(HL)] than in the complex [CuL2(HL)]. The calculated one-electron energy level diagrams for the complex dimers 2ML2 (M ) CoII, NiII, and CuII) are also shown in Figure 4. From Figure 4, the cobalt(II) and copper(II) dimers are spin-polarized to a triplet ground state, although the nickel(II) dimer still has a singlet ground state because of its nonspin polarization. The unpaired electrons of 2CoL2 exist in the molecular orbitals 75agV and 75auV. The contour maps of the 75agV and 75auV of 2CoL2 are shown in Figure 7. It is worth noting that the 75agV and 75auV of 2CoL2 can be approximately represented by superposition of the 38agV of the monomer CoL2 on another 38agV symmetrically and asymmetrically for the Ci symmetry, respectively,23 from the resemblance of the contour map between the 75agV or 75auV of 2CoL2 and the 38agV of CoL2. On the other hand, the unpaired electrons of 2CuL2 are in the molecular orbitals 76agv and 76auv. Comparing their contour maps in Figure 8 with the contour map in Figure 5c, the MOs 76agv and 76auv of the dimer 2CuL2 are also well represented by superposing the MO 39agv with an unpaired electron of the monomer CuL2 on another 39agv symmetrically and asymmetrically with respect to the Ci symmetry, respectively.24 The contour map of the differential spin density of 2CoL2 is shown in Figure 9a. In contrast with the case of the monomer CoL2 (Figure 6a), the differential spin density of 2CoL2 increases in the region where the monomer units overlap with each other. On the basis of the results from Mulliken's population analysis

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TABLE 1: Atomic Populations and Differential Atomic Populations of the Monomer ML2 and the Dimer 2ML2 (M ) CoII, NiII, and CuII)a,b atom

CoL2

2CoL2

NiL2

2NiL2

CuL2

2CuL2

M(1,28)

26.712 46 -0.305 00 7.131 79 -0.038 49

26.711 20 -0.342 56 7.156 19 -0.014 81 7.104 94 -0.072 98 6.024 70 -0.018 62 5.970 76 -0.021 04 8.440 47 -0.045 21 8.372 99 -0.181 59 7.519 54 -0.033 76 7.468 20 -0.104 50 7.125 77 -0.018 71 7.104 37 -0.065 33 6.031 02 +0.000 58 5.971 10 -0.006 46 8.370 41 -0.009 19 8.351 21 -0.057 79 7.607 82 -0.004 78 7.588 60 -0.020 50 0.689 31 +0.003 41 0.673 25 +0.002 45 0.830 36 +0.001 70 0.815 78 +0.005 60 0.394 51 +0.000 57 0.397 53 +0.001 93 0.320 29 +0.000 15 0.326 88 +0.000 50 0.833 04 +0.000 24 0.799 72 +0.000 68

27.590 26 0 7.143 76 0

27.590 42 0 7.194 88 0 7.156 40 0 5.954 52 0 5.946 26 0 8.396 32 0 8.362 40 0 7.538 76 0 7.537 12 0 7.099 64 0 7.117 52 0 5.989 94 0 5.945 76 0 8.417 16 0 8.409 54 0 7.396 70 0 7.392 56 0 0.653 90 0 0.668 62 0 0.800 44 0 0.785 82 0 0.357 06 0 0.342 94 0 0.807 40 0 0.778 58 0 0.667 28 0 0.692 10 0

28.490 99 +0.173 07 7.247 73 +0.088 11

28.586 34 +0.187 00 7.270 60 +0.082 56 7.211 65 +0.088 13 6.042 45 -0.00397 6.024 14 -0.006 72 8.436 64 +0.025 88 8.404 32 +0.027 16 7.918 33 -0.000 95 7.851 97 -0.000 83 7.166 47 +0.103 23 7.180 94 +0.121 36 6.045 11 -0.013 25 5.973 49 -0.014 77 8.468 41 +0.179 83 8.449 92 +0.245 92 7.563 53 -0.008 25 7.511 16 -0.010 98 0.603 23 -0.001 65 0.599 93 -0.001 67 0.190 78 -0.000 04 0.165 46 -0.000 10 0.400 17 +0.000 03 0.421 89 +0.000 03 0.710 80 +0.000 58 0.765 62 +0.000 76 0.516 99 +0.000 33 0.519 65 +0.000 39

N(2,42) N(15,29) C(4,44)

5.967 56 -0.040 06

C(17,31) O(3,43)

8.406 58 -0.100 26

O(16,30) N(5,45)

7.530 12 -0.068 28

N(18,32) N(6,46)

7.116 03 -0.053 09

N(19,33) C(8,48)

5.988 12 -0.005 28

C(21,35) O(7,47)

8.360 59 -0.041 33

O(20,34) N(9,49)

7.573 11 -0.017 85

N(22,36) H(10,50)

0.670 77 +0.003 09

H(23,37) H(11,51)

0.848 62 +0.000 88

H(24,38) H(12,52)

0.393 94 +0.012 02

H(25,39) H(13,53)

0.321 05 +0.000 45

H(26,40) H(14,54) H(27,41)

0.835 48 +0.000 74

5.949 48 0 8.388 70 0 7.505 02 0 7.108 68 0 5.968 62 0 8.422 72 0 7.400 54 0 0.672 98 0 0.796 02 0 0.399 94 0 0.804 70 0 0.643 72 0

6.024 89 -0.004 77 8.410 01 +0.027 11 7.898 19 -0.001 55 7.176 04 +0.112 06 6.010 96 -0.015 14 8.464 31 +0.218 05 7.522 51 -0.014 41 0.612 74 -0.001 72 0.200 00 -0.000 06 0.422 58 +0.000 02 0.743 23 +0.000 67 0.521 30 +0.005 08

a Differential atomic populstion is defined as N v-N V, where N v and N V are atomic populations for up spin (v) and down spin (V) on an atom x x x x X, respectively. b Atomic population and differential atomic population are given on the upper and the lower line for each atom, respectively. The values for the equivalent atoms in the monomers are omitted for simplicity.

in Table 1, the differential spin density increases in the cobalts Co1 and Co28, in the nitrogens N18 and N32 occupying apical sites of the cobalts, and in the nitrogens N15 and N29 coordinating to the cobalts on the planes of the monomer units, generating two pathways to couple the cobalt atoms Co1 and Co28 together (one is Co1-N32-C31-N29-Co28, and the other is Co28-N18-C17-N15-Co1). Further, although the differential spin density of 2CoL2 decreases relatively in the portion where the two monomer units do not overlap with each other, the large ∆FvV value (-0.033 76e) is still maintained on N5 and N45, which are free in the dimer model, but occupy apical sites of the neighboring complex molecules in the stack chain. In contrast, the differential spin density of the copper(II) dimer 2CuL2 is principally characterized by the localization

of differential spins near each copper atom and the atoms surrounding the copper in Figure 9b. This is very similar to the character of the monomer CuL2 described earlier (Figure 6b). It is worth noting the very low density of differential spins on N18 and N32 (-0.000 83e in Table 1) occupying apical sites of the coppers and on N5 and N45 (-0.000 95e in Table 1) at apical sites of the neighboring complex molecules in the stack chain of the real system. Thus, this characteristic of the differential spin density for 2CuL2 clearly suggests that two monomer units are rather isolated from each other owing to the lack of pathways for superexchange between the metals, which exist in the case of 2CoL2.

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Figure 6. Contour maps of the differential spin density ∆FvV of (a) CoL2 on the plane Z ) 1.0 au and (b) CuL2 on the plane Z ) 0 au. Solid, dotted, and broken lines denote positive, negative, and zero, respectively. Figure 5. Contour maps of molecular orbitals of the monomers ML2: (a) the POMO 38agV of CoL2 on the plane Z ) 1.0 au, (b) the HOMO 37 auv(V) of NiL2 on the plane Z ) 1.0 au, and (c) the POMO 39agv of CuL2 on the plane Z ) 0 au. Solid, dotted, and broken lines denote positive, negative, and zero, respectively.

Discussion We have revealed that the divalent metal complexes with diaminoglyoxime [ML2(HL)] (M ) CoII, NiII, and CuII), in spite of their resemblance in stacking mode, show quite different magnetic behaviors: the CoII complex exhibits ferromagnetic interactions along the stacking chain, while the NiII complex is diamagnetic, and the CuII complex is almost paramagnetic. In order to understand the nature of the magnetism of these diaminoglyoxime complexes, we investigate the magnetic interactions between monomer units in the dimer model 2ML2. The magnetic properties of a binuclear complex a-b, where a (or b) denotes a monomeric fragment with Sa ) 1/2 (or Sb ) 1/ ), have been qualitatively examined on the basis of the MOs 2 φa and φb of unpaired electrons for the fragments a and b, respectively.1b,25 Kahn et al. reported that the singlet-triplet splitting of such a system results from competition between two antagonist terms; one (JF), a ferromagnetic contribution, is proportional to the two-electron exchange integral between φa and φb, and the other (JAF), an antiferromagnetic contribution, is proportional to the square of the overlap integral of the MOs.1b The JF term depends mainly on the zones of strong overlap

density φaφb, irrespective of their sign. The JAF term, on the other hand, depends on the balance between a positive and a negative zone of the overlap density. According to this concept, we can roughly predict that the dimer 2CoL2 is more ferromagnetic than the dimer 2CuL2 and that the 2CuL2 is almost paramagnetic because JF(2CuL2) , JF(2CoL2) and JAF(CuL2) ≈ JAF(2CoL2) ≈ 0, provided that the MOs φa and φb are the MO 38agV for CoL2 and the MO 39agv for CuL2. In this study, we undertake to evaluate directly the magnetic interactions between the monomer units in the dimer 2ML2, on the basis of the molecular orbital calculations of the monomer ML2 and the dimer 2ML2, using the unrestricted DV-XR method. Generally, magnetism results from the exchange interactions of electrons. According to the XR method, the spin splitting of the kth molecular orbital φA,k of a molecule A, ∆A,kvV°, is approximately expressed as a difference of exchange interaction between the up spin and the down spin, as in eq 3.26

∆A,kvV° ≈ -3R(3/4π)1/3{〈φA,kv|FAv1/3|φA,kv〉 〈φA,kV|FAV1/3|φA,kV〉} (3) As a minute variation of the exchange interaction can be described as a deviation of the spin splitting under a small perturbation, as shown in Appendix A, we can safely estimate the exchange interaction in a molecular system from the spin splitting. Thus, we have concentrated our discussion on the

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Figure 7. Contour maps of the POMOs (a) 75agV and (b) 75auV of the dimer 2CoL2 on the plane, the Z value of which is by +1.0 au larger than that of the plane of the monomer unit with Z < 0. Solid, dotted, and broken lines are the same as in Figure 5.

spin splitting of the POMOs of the dimer 2ML2 in order to examine the magnetism of 2ML2. We consider a dimer A-B belonging to Ci symmetry with paramagnetic monomer units A (SA ) 1/2) and B (SB ) 1/2) weakly interacting with each other as the model for 2ML2 (M ) CoII and CuII). Assuming that the POMOs of the dimer, φA-B,ζ, are expressed by superposition of the POMOs of the monomers A and B, φA,l and φB,l, and that the spin density of the dimer, FA-Bv (or FA-BV), is also described by superposition of the spin densities of the monomers A and B, FAv (or FAV) and FBv(or FBV), eqs 4 and 5 hold for the up spin.

φA-B,ζv ≈ φA,lv + φB,lv

(4)

FA-Bv ≈ FAv + FBv

(5)

The symmetry of the dimer A-B requires the following relations for the up spin,

φA,lv ) (R(φB,lv)

(6)

FAv ) (R(FBv)

(7)

where R denotes the symmetry operation for the center of symmetry. Similar relations hold for the down spin. The cobalt(II) and the copper(II) dimer 2ML2 (M)Co, and Cu) satisfies eqs 4-7, as described in the preceding section. According to Appendix B, the magnetic interaction between

Figure 8. Contour maps of the POMOs (a) 76agv and (b) 76auv of the dimer 2CuL2 on the plane of the molecular unit with Z < 0. Solid, dotted, and broken lines are the same as in Figure 5.

the monomer units in the dimer A-B, JA-B, can be estimated from the difference of the spin splitting of the POMO between the monomer and the dimer, as expressed by eq 8,

JA-B ) (∆A-B,ζ(+)vV - ∆A,kvV°) + (∆A-B,ζ(-)vV - ∆A,kvV°) (8) where + and - indicate the symmetry and the asymmetry for the operation R, respectively. This relationship clearly shows that the dimer A-B is ferromagnetic if JA-B > 0, paramagnetic if JA-B ) 0, and antiferromagnetic if JA-B < 0. For the cobalt(II) dimer 2CoL2 which has the POMOs 75ag and 75au, the magnetic interaction J2Co is expressed by the following equation from eq 8,

J2Co ) (∆2Co,75auvV - ∆Co,38agvV°) + (∆2Co,75agvV - ∆Co,38agvV°) (9) and similarly the magnetic interaction for 2CuL2 is expressed by eq 10.

J2Cu ) (∆2Cu,76auvV - ∆Cu,39agvV°) + (∆2Cu,76agvV - ∆Cu,39agvV°) (10) The results calculated from eqs 9 and 10 are collected in Table 2. These results show that 2CoL2 has a much larger J value than 2CuL2. However, it must be mentioned that the calculated

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J. Phys. Chem. B, Vol. 101, No. 23, 1997 4505

Figure 9. Contour maps of the differential spin density ∆FvV of (a) 2CoL2 on the plane, the Z value of which is by +1.0 au larger than that of the plane of the monomer unit with Z < 0 and of (b) 2CuL2 on the plane of the monomer unit with Z < 0. Solid, dotted, and broken lines are the same as in Figure 6.

TABLE 2: Magnetic Interactions between Monomer Units in the Dimer 2ML2 (M ) CoII and CuII)

a

dimer

J(eV)a

2CoL2 2CuL2

0.051 25 0.002 84

These values were obtained from eqs 9 and 10.

J values are much larger than those expected from the experiments (the observed value of the ferromagnetic intrachain interactions for [CoL2(HL)] is 13.5K, i.e., 0.001 16 eV). This is probably due to a larger variation of spin density in the dimer model than in the real system. It should be mentioned that 2CoL2 has a higher differential spin density in the portion where the monomeric units overlap with each other. In the real stack chain, however, the differential spin density on N5 (or N32) in Figure 1 has to be the same as that on N18 (or N45), because of their equivalence in structure. Thus, considering that the spin splitting depends largely on the differential spin density as described above, the J value for the dimer could be larger than that expected in the real system. Taking this into account, it could be concluded that the J values for the dimers in Table 2 are satisfactorily consistent with the experimental findings that [CoL2(HL)] is a linear ferromagnet and [CuL2(HL)] is paramagnetic. The CoL2 has the POMO 38agV developing along the direction perpendicular to the molecular plane because of the π character

consisting of the 3dzx (Co) and 2pz bases (C, N, and O on the ligands). The 2CoL2 has the POMOs 75agV and 75auV, which are expressed by superposition of the POMO 38agV of the monomer units on another 38agV symmetrically and asymmetrically, respectively, implying larger magnetic interactions between the monomeric units. In comparison with the monomer, the dimer exhibits a marked increase of the differential spin density in the overlapping zone of the monomeric units, as clearly shown in Figure 9a (see the values on N18 (N32), C21 (C35), N15 (N29), O16 (O30), and Co1 (Co28) in Table 1). In particular, the differential spin density on N18 (or N32), which occupies an apical site of Co in each monomeric unit, is more than 1.5 times larger than that on the relevant N in the monomer. Accordingly, this strongly suggests that unpaired spins in the dimer 2CoL2 are coupled parallel to each other through the strong exchange interaction between the 3dzx orbitals of Co1 (and Co28) and the 2pz orbitals of N32 (and N18) at an apical site of each Co atom. Probably a similar mechanism causes the linear ferromagnetism along the stack chain of the cobalt complex [CoL2(HL)]. Conversely, the copper(II) monomer CuL2 has the POMO 39agv developing on the molecular plane owing to a π character with the dxy (Cu), and the 2px and 2py bases (C, N, and O on the ligands) (Figure 5c). Furthermore, the differential spin density of CuL2 localizes intensively on the central Cu and on the atoms surrounding the metal, N2 (N15), N6 (N19), O3 (O16), and O7 (O20), not on N5 (N18) as coordination sites toward the neighboring complexes in the real stack chain. The dimer 2CuL2 has the POMOs 76agv and 76auv, which are characterized by the superposition of the POMO 39agv of the monomer on another 39agv symmetrically and asymmetrically, respectively, as described before. Thus, in comparison with the dimer 2CoL2, the magnetic interaction between the monomeric units in 2CuL2 is probably very small because of the orbital mode of the 39agv developing on the molecular plane. This prediction is also supported by the differential spin density of 2CuL2 in Figure 9b. The differential spin density is extremely low on N18 and N32 (-0.000 83e in Table 1) occupying apical sites of Co28 and Co1, respectively and it fails to channel the superexchange between the Cu metals. Figure 9b well illustrates that the spins in 2CuL2 are isolated from each other, declaring that the dimer is virtually paramagnetic. A similar situation may occur in the real stack chain in the crystalline phase of [CuL2(HL)]. The magnetism of the nickel system [NiL2(HL)] can be predicted from the calculated results of the monomer NiL2. The monomer NiL2 has only nondegenerated molecular orbitals because of its symmetry Ci. Such a molecular system with an even number of electrons usually favors nonspin polarization. We can conclude from these results that the magnetic behaviors of the divalent metal complexes with diaminoglyoxime [ML2(HL)] (M)CoII, NiII, and CuII) with a slipped-stack mode are precisely predicted from a comparison of the spin splitting of the POMO of the monomer complex ML2 and the dimer complex 2ML2 which are equivalent in structure to the relevant moieties in the real stack chain. In addition, the differential spin density of the dimer 2ML2 can satisfactorily predict the mechanism of superexchange in the stack chain. Therefore, the method developed in this study will play an important role in the design of novel organomagnetic materials with a variety of stack modes. Acknowledgment. We are grateful to Prof. H. Adachi (Kyoto University) for his kind instruction in the concept of the DV-XR method. We thank Dr. N. Higuchi (Electrotechnical Laboratory) for the measurement of magnetic susceptibility by the SQUID.

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Appendix A According to the XR method, the spin splitting of the kth molecular orbital φA,k for a molecule A, ∆A,kvV°, is approximately expressed in the following equation.26

monomer units A and B, respectively. Similar formulas are written for the down spin. The symmetry of the dimer requires the following relations with respect to the symmetry operation for the center of symmetry R,

φB,l v ) (R(φA,lv),

∆A,kvV° ≈ -3R(3/4π)1/3{〈φA,kv|FAv1/3|φA,kv〉 〈φA,kV |FAV1/3|φA,kV〉} (A1) Let us derive the formula of spin splitting ∆A,kvV, when the molecular orbital and spin density vary minutely under a small perturbation as follows,

φA,k f φA,k + δφA,k

FAv ) (R(FBv),

From eq A1,

Also the following relations hold,

〈φA,mv|φB,nv〉 ) 0,

Expanding the the term (FA + δFA)1/3,

(A10)

Substituting eqs A6 and A7 into eq A11 and using eqs A8A10, eq A11 is rewritten as eq A12.

(FA + δFA)1/3 ) {FA(1 + δFA/FA)}1/3

∆A-B,ζvV° ≈ -3R(3/4π)1/3{〈φA,lv|FAv1/3|φA,lv〉 -

)FA1/3 + (1/3)δFA/FA2/3 + ...

〈φA,lV|FAV1/3|φA,lV〉} ≈ ∆A,lvV° (A12)

(A3)

By use of eq A3, eq A2 is rewritten as eq A4.

∆A,kvV ≈ ∆A,kvV° -3R(3/4π)1/3[2{〈δφA,kv | FAv1/3|φA,kv〉 〈δφA,kV | FAV1/3|φA,kV〉} + {〈φA,kv | δ(FAv1/3) |φA,kv〉 -

Thus the spin splitting of the POMO in the hypothetical dimer is approximately equal to that of the relevant monomer itself. Next, let us consider the dimer system with small interactions between the monomer units. We assume that the POMO φA-B,ζ and spin density FA-Bv (FA-BV) are expressed by eqs A13 and A14 for the up spin, respectively.

〈φA,kV | δ(FAV1/3)|φA,kV〉}] + ... ≈ ∆A,kvV° -3R(3/4π)

〈φA,mV|φB,nV〉 ) 0

〈φA-B,ζ°V|FA-BV°1/3|φA-B,ζ°V〉} (A11)

δFAV)1/3|φA,kV + δφA,kV〉} (A2)

+ δ(FA ) + ...

(A9)

-3R(3/4π)1/3{〈φA-B,ζ°v | FA-Bv°1/3|φA-B,ζ°v〉 -

δFAv) |φA,kv + δφA,kv〉 - 〈φA,kV + δφA,kV|(FAV + 1/3

)FA

FAV ) (R(FBV)

∆A-B,ζvV° ≈

∆A,kvV ≈ -3R(3/4π)1/3{〈φA,kv + δφA,kv|(FAv +

1/3

(A8)

where φA,mv (φA,mV) and φB,nv (φB,nV) are the molecular orbitals of the monomers A and B, respectively. The spin splitting of the POMO φA-B,ζ°, ∆A-B,ζvV°, is written as eq A11 on the basis of eq A1.

FA f FA + δFA

1/3

φB,lV ) (R(φA,lV)

1/3

δ{〈φA,kv | FAv |φA,kv〉 -

φA-B,ζv ) φA-B,ζ°v + δφA-B,ζ°v

(A13)

FA-Bv ) FA-B°v + δFA-B°v

(A14)

1/3

〈φA,kV | FAV1/3|φA,kV〉} + ... (A4) Thus,

∆A,kvV - ∆A,kvV° ≈ -3R(3/4π)1/3 δ{〈φA,kv | FAv1/3|φA,kv〉 〈φA,kV | FAV1/3|φA,kV〉} + ... (A5)

Similar formulas are written for the down spin. The spin splitting of the POMO φA-B,ζ , ∆A-B,ζvV, is represented as eq A15, by use of eqs A4 and A12.

∆A-B,ζvV ≈ ∆A-B,ζvV° -3R(3/4π)1/3 δ{〈φA-B,ζ°v|FA-Bv°1/3|φA-B,ζ°v〉 〈φA-B,ζ°V|FA-BV°1/3|φA-B,ζ°V〉}

It is shown from eq A5 that the variation of the spin splitting is equal to that of the exchange interaction under a small perturbation.

≈ ∆A,lvV° -3R(3/4π)1/3 × δ{〈φA-B,ζ°v|FA-Bv°1/3|φA-B,ζ°v〉 〈φA-B,ζ°V|FA-BV°1/3|φA-B,ζ°V〉} (A15)

Appendix B We consider a hypothetical dimer A-B belonging to Ci symmetry and consisting of monomer units A and B without any interaction between the monomer units. For this system, the POMOs, φA-B,ζ°, and the spin density FA-B°v (FA-B°V) are written as eqs A6 and A7 for the up-spin,

φA-B,ζ°v ) (1/2)1/2 (φA,lv + φB,lv)

(A6)

FA-B°v ) FAv + FBv

(A7)

where φA,lv and φB,lv are the POMOs for the up spin of the

Thus,

∆A-B,ζvV - ∆A,lvV° ≈ -3R(3/4π)1/3 δ{〈φA-B,ζ°v|FA-Bv°1/3|φA-B,ζ°v〉 〈φA-B,ζ°V|FA-BV°1/3|φA-B,ζ°V〉} (A16) This equation means that the magnetic interaction between the monomer units in the dimer can be estimated from the difference of the spin splitting of the POMO between the dimer and the

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J. Phys. Chem. B, Vol. 101, No. 23, 1997 4507

monomer. Therefore, the magnetic interaction of the dimer JA-B is written as

JA-B ) Σζ (∆A-B,ζvV - ∆A,lvV°)

(A17)

where the summation is of all the POMOs. References and Notes (1) (a) Kahn, O.; Pei, Y.; Verdaguer, M.; Renard, J. P.; Sletten, J. J. Am. Chem. Soc. 1988, 110, 782. (b) Kahn, O. Angew. Chem., Int. Ed. Engl. 1985, 24, 834. (2) (a) Caneschi, A.; Gatteschi, D.; Renard, J. P.; Rey, P.; Sessoli, R. Inorg. Chem. 1989, 28, 3314. (b) Caneschi, A.; Gatteschi, D.; Renard, J. P.; Rey, P.; Sessoli, R. Inorg. Chem. 1989, 28, 1976. (c) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. Acc. Chem. Res. 1989, 22, 392. (3) Inoue, K.; Iwamura, H. J. Am. Chem. Soc. 1994, 116, 3173. (4) Tamaki, H; Zhong, Z. J.; Matsumoto, N.; Kida, S.; Koikawa, M.; Achiwa, N.; Hashimoto, Y.; O ˆ kawa, H. J. Am. Chem. Soc. 1992, 114, 6974. Decurtins, S.; Schmalle, H. W.; Schneuwly, P.; Ensling, J.; Gu¨tlich, P. J. Am. Chem. Soc. 1994, 116, 9521. (5) McConnell, H. M. Proc. Robert A. Welch Found. Conf. Chem. Res. 1967, 11, 144. (6) (a) Miller, J. S.; Calabrese, J. C.; Rommelmann, H.; Chittipeddi, S. R.; Zhang, J. H.; Reiff, W. M.; Epstein, A. J. J. Am. Chem. Soc. 1987, 109, 769. (b) Manriquez, J. M.; Yee, G. T.; McLean, R. S.; Epstein, A. J.; Miller, J. S. Science 1991, 252, 1415. (7) Oshio, H. Inorg. Chem. 1993, 32, 4213. Oshio, H.; Ino, E.; Mogi, I.; Ito, T. Inorg. Chem. 1993, 32, 5697. (8) For example see: Yoshizawa, K.; Hoffmann, R. J. Am. Chem. Soc. 1995, 117, 6921. (9) Bekaroglu, O ¨ .; Sarisaban, S.; Koray, A. R.; Ziegler, M. L. Z. Naturforsch. 1977, B32, 387. (10) (a) Endres, H.; Jannack, T.; Prickner, B. Acta Crystallogr. 1980, B36, 2230. (b) Herbstein, F. H.; Marsh, R. E. Acta Crystallogr. 1982, B38, 1051. (11) Endres, H.; Genc. N.; No¨the, D. Acta Crystallogr. 1983, C39, 701.

(12) Hirotsu, T.; Takagi, N.; Sakakibara, J.; Katoh, S.; Senoˆ, M. J. Chem. Soc., Chem. Commun. 1990, 1631. (13) Averill, F. W.; Ellis, D. E. J. Chem. Phys. 1973, 59, 6412. Ellis, D. E.; Adachi, H.; Averill, F. W. Surf. Sci. 1976, 58, 497. Rosen, A.; Ellis, D. E.; Adachi, H.; Averill, F. W. J. Chem. Phys. 1976, 65, 3629. Adachi, H.; Tsukada, M.; Satoko, C. J. Phys. Soc. Jpn. 1978, 45, 875. Adachi, H.; Shiokawa, S.; Tsukada, M.; Satoko, C.; Sugano, S. J. Phys. Soc. Jpn. 1979, 47, 1528. (14) Fischer, E. Ber. 1889, 22, 1930. Pearse, G. A., Jr.; Pflaum, R. T. Anal. Chem. 1960, 32, 213. (15) The Chemical Society of Japan, Ed. Kagaku Binran-Kisohen II; Maruzen: Tokyo, 1984; p. II-508 (Japanese). (16) The program of the DV-XR method was offered from Prof. H. Adachi (Kyoto University). (17) Baker, G. A., Jr.; Rushbrooke, G. S.; Gilbert, H. E. Phys. ReV. A 1964, 135, 1272. (18) Swank, D. D.; Landee, C. P.; Willet, R. D. Phys. ReV. B 1979, 20, 2154. (19) Leggett, D. J. The Determination of Formation Constants-An Overview of Computational Methods for Data Processing. In Computaional Methods for the Determination of Formation Constants; Leggett, D. J., Ed.; Plenum Press: New York, 1985; pp 5-9. (20) This calculated value is compensated by the contribution from Schiff bases. (21) For example see: Lubitz, W.; Winscom, C. J.; Diegruber, H.; Moseler, R. Z. Naturforsch. 1987, A42, 970. (22) Kahn, O.; Sikorav, S.; Gouteron, J.; Jeannin, S.; Jeannin, Y. Inorg. Chem. 1983, 22, 2877. Caneschi, A.; Gatteschi, D.; Sessoli, R. Inorg. Chem. 1993, 32, 4612. (23) The 75agv and 75auv of 2CoL2 can be approximately expressed with the 38agv of CoL2 in the similar manner. (24) The 76agV and 76auV of 2CuL2 can be approximately expressed by use of the 39agV of CuL2 in the similar manner. (25) Kahn, O.; Briat, B. J. Chem. Soc., Perkin Trans. 2 1976, 72, 268. Girerd, J. -J.; Charlot, M. -F.; Kahn, O. Mol. Phys. 1977, 34, 1063. Tola, P.; Kahn, O.; Chauvel, C.; Coudanne, H. NouV. J. Chim. 1977, 1, 467. (26) Slater, J. C. The Calculation of Molecular Orbitals; Wiley: New York, 1979; p 8.